Properties such as those listed above are used to evaluate the quality or performance of a fluid such as paint. For example, the visual aspect given to a painted surface for a determined illumination depends on the colour of the paint used to paint the surface. Another important property is the opacity, or hiding power, of the paint which indicates the capacity of the paint, for a determined paint film thickness in the dry state, to hide the background colour of the surface on which it is painted.
Both colour and opacity are the objects of product control in paint factories, where visual or spectrophotometric techniques are presently used to analyse the paint. The techniques of the state of the art utilise a basically manual process, where paints are mixed, then sprayed onto a substrate and allowed to dry or are cured after which they are subjected to analysis. This process is extremely time consuming, the standard time taken to analyse a paint being of the order of 36 hours.
Paint manufacturing processes normally use pigment pastes which, when mixed in sufficient proportions, result in the final desired colour. The opacity of the paint is adjusted usually by the addition of resin (transparent varnish) to the mixture, in proportion to the degree of opacity desired.
There are, at present, two methods available for supplying paint to the market: the traditional so-called "factory pack" method, where a paint is produced and has its physical properties (colour, opacity and viscosity) adjusted in the factory, using the techniques mentioned above; and the commercial "mixing" method, where the paint is produced, at the point of sale, by mixing a number of coloured bases--paints having a specified colour such as is used in the CMYK (cyan, magenta, yellow and black) system--in specified proportions to produce the desired colour. This second method brings advantages to both the supplier and the client, making it possible for a large quantity of standard colours and shades to be offered from a reduced stock of coloured bases.
Obviously, these bases must be rigorously controlled with regard to their colour and shade, so that, for a specified proportion of paints mixed together, the resulting paint colour and shade does not vary significantly from one batch of paint to another.
It should also be possible to obtain "factory pack" paints using the "mixing" method, but this relies on the coloured bases having their colorific properties strictly controlled.
In the "mixing" method, the coloured bases (mixtures of pigments, resins, solvents and other additives) have their hues and saturations adjusted by so-called "cutting" of the base by mixing it with a determined proportion of a standard base--a white, black or green base. Once the coloured base has been mixed, it is applied to a surface and allowed to dry or is cured, after which the colorific properties are measured. Comparison of these measured properties with those of a standard base, provide the parameters for whatever adjustment is needed.
This method of comparing the coloured base with a standard base is necessary due to the inherent variability of the batches of pigments supplied to paint manufacturers, as well as to variations in the base fabrication process. For example, one coloured base may have a tinting strength greater than the same coloured base from a different batch. In this case, if both bases are cut with the same proportion of a standard white base, the coloured base having the higher tinting strength will develop a more intense colour.
The technique of mixing or cutting a coloured base with a standard white base, so called desaturation, is necessary because, in their natural state, the concentration of the pigments is such that there is not a sufficient distinction between them in terms of tinting strength and hue, they are chromatically saturated. Therefore, cutting of these chromatically saturated pigments with a standard white base has a "zoom" effect, allowing the various properties of the paint to be measured effectively.
One of the problems with the cutting technique is that a standard white base has to be maintained, and, as with the coloured bases, this base comprises pigments dispersed in resins, solvents and other additives, which in general are not stable. These dispersions of pigments are susceptible to the problems of reaglomoration, sedimentation, evaporation of the solvent--with a consequent increase in concentration--contamination, not to mention problems of variability with atmospheric conditions. Therefore, as with the coloured bases, a pheric conditions. Therefore, as with the coloured bases, a problem arises with respect to the calibration of the properties of the standard base.
In the present state of the art, the standard white base is standardised with respect to either a standard black or a standard green base, which in turn has been standardised with respect to a prior standard white base, and so on, ad infinitum.
A further problem that arises with respect to the present state of the art in the measurement of paint properties is that, as mentioned above, the colorific properties of the paint are measured after it has been applied to a surface and either cured or dried. Thus, the perceived properties of the paint rely on the thickness of the layer applied to the surface, and to the properties of the surface itself, which means that, in order to avoid misinterpretation of the results, due to the influence of the colour and hue of the surface, it is essential that the thickness of the paint applied to the surface be rigorously controlled.
Other sources of error that may be introduced into the measurement of paint properties are: in the weighing of the components of the paint; the pressure of the spray used to apply the paint to the surface; the drying temperature; the method of preparation of the surface; the relative humidity of the air; etc..
There are a number of prior art documents which describe devices and processes for measuring the physical properties of paints and other fluids, however, non of these devices or processes allow the true automation of the paint or fluid production process, requiring manual intervention in order to produce either a reflection spectrum or a transmission spectrum, but not both, of the fluid under analysis. There follows a brief description of some such prior art documents:
DE 25 25 701 describes an apparatus for measuring the colour of paint in its liquid form, by forming a film of paint that is irradiated and the reflected radiation is analysed spectrophotometrically. The film is formed by allowing a stream of paint to impinge on a disc which is spinning about a horizontal axis. The paint forms a film as it runs down the disc under the force of gravity due to the centrifugal force provided by the spinning disc. It is not possible to adjust automatically the thickness of the film of paint in order to provide consistent optimised measurements of the paint properties, and the apparatus is only suitable for making measurements of the reflection spectrum of the paint.
EP 0 304 172 describes a method and apparatus for measuring the colour properties of a paint by irradiating a sample volume of the paint, which is subjected to shearing forces and turbulence, and analysing the radiation reflected from the sample. According to this document, the application of shearing forces to the volume of paint under analysis is advantageous in that deflocculation of the pigments within the paint does not occur. However, the apparatus described in this document is unsuitable for the measurement of the transmission spectrum of a paint, since a film of paint is not formed, and is unsuited to use on-line in a paint production process.
JP 02059627 describes a method and device for colorimetric analysis of a paint by forming a liquid film and measuring its "spectral reflectance". The film is formed by inserting a bar into a paint reservoir and lifting it out to pull a film of paint out of the reservoir by means of surface tension. The apparatus is unsuitable for use on-line in a paint production process, and can be used only for making measurements of the reflection spectrum, it being impossible to control the thickness of the film.
EP 0 302 009 describes a fluid sampling cell for use in measuring the transmission spectrum of a high temperature fluid. The sampling cell comprises a sampling region through which a fluid is allowed to flow, and the sampling region has a film forming means comprising two windows, one opposite the other, between which a film of fluid is formed. The film of fluid is irradiated through one of the windows and the transmitted radiation which passes through the other window is directed to a spectrophotometer for analysis.
This document is specifically concerned with the problem of how to keep the windows a specified distance apart during measurement of the. transmission spectrum of the fluid film, in order to avoid measurement errors caused by changes in the separation of the windows due to heat expansion of their holders when high temperature fluids are being analysed. This is achieved by supplying at least one of the windows with raised projections which are pressed against the other window during measurement to ensure a fixed separation between the windows equal to the height of the raised projections. The apparatus described in this document is therefore suitable only for performing transmission spectrum analysis of a fluid whose properties are invariable, and would not be suitable for use in a paint manufacturing process, where batches of paints having different physical properties need to be analysed.
Finally, U.S. Pat. No. 3,740,156 describes a photometric analyser sampling cell, for performing transmission spectrum analysis of molten or liquid plastics materials, the cell comprising a film forming means for forming a fluid film having a prespecified thickness in a sampling region, by trapping a sample of fluid between two coaxial windows, one of which is fixed and the other of which is moveable. The film is irradiated through one of the windows and the transmitted radiation passes through the other window and is directed to a photometric analyser. The apparatus described in this document is almost identical to that described in EP 0 302 009, the windows being held a fixed distance apart during measurement of the transmission spectrum of the fluid sample, the only difference being the manner in which this fixed distance is achieved.